The present invention relates to a liquid crystal display apparatus using a liquid crystal device as a light value for use in flat-panel displays, projection displays, etc., and a driving method for the liquid crystal display apparatus.
A twisted nematic (TN) liquid crystal has widely been used conventionally as a material for flat-panel displays as described by M. Schadt and W. Helfrich, xe2x80x9cApplied Physics Lettersxe2x80x9d, Vol. 18, No. 4 (Feb. 15, 1971), pp. 127-128. The TN liquid crystal is used in an active matrix-type liquid crystal device (panel) in combination with switching elements such as thin film transistors (TFTs). The active matrix-type liquid crystal device is free from a problem of cross-talk since each pixel is provided with a switching element and is produced with high productivity with respect to that having a size (diagonal length) of 10-17 in. with quick a progress of production technique in recent years.
However, the above-mentioned liquid crystal device using the TN liquid crystal has been accompanied with problems such as a slower response speed and a narrower viewing angle in order to well display clear motion (picture) images.
In order to solve the problems, various alignment modes including an optically compensated bend or birefringence (OCB) mode for improving a response speed, and In-Plain Switching mode and MVA (Multi-domain Vertical Alignment) mode for improving a viewing angle have been developed and proposed.
Further, in order to solve the problems of the conventional TN liquid crystal devices, a liquid crystal device using a chiral smectic liquid crystal exhibiting bistability has been proposed by Clark and Lagerwall (Japanese Laid-Open Application (JP-A) 56-107216, U.S. Pat. No. 4,367,924). As the liquid crystal exhibiting bistability, a ferroelectric liquid crystal having chiral smectic C phase (SmC*) or H phase (SmH*) is generally used. Such a ferroelectric liquid crystal provides a very quick response speed because it causes inversion switching of liquid crystal molecules based on their spontaneous polarizations. In addition, the ferroelectric liquid crystal assumes bistable state showing a memory characteristic.
In recent years, an anti-ferroelectric liquid crystal exhibiting tristable state has been proposed by (chandani, Takezoe et al. (xe2x80x9cJapanese Journal of Applied Physicsxe2x80x9d, vol. 27 (1988), pp. L729-). The anti-ferroelectric liquid crystal also provides a very quick response speed similarly as in the ferroelectric liquid crystal.
As another type of the anti-ferroelectric liquid crystal, there has been recently proposed a chiral smectic liquid crystal providing a V-character shaped response characteristic (voltage-transmittance characteristic) which is advantageous for gradational image display and is free from hysteresis (e.g., xe2x80x9cJapanese Journal of Applied Physicsxe2x80x9d, Vol. 36 (1997), pp. 3586-). Further, an active matrix-type liquid crystal device using such a chiral smectic liquid crystal providing the V-shaped voltage-transmittance characteristic has also been proposed (JP-A 9-50049).
As described above in order to provide a liquid crystal display apparatus with a high-speed responsiveness and a good gradational display characteristic, liquid crystal displays of the above-mentioned OCB-mode and anti-ferroelectric liquid crystal materials have been extensively researched and developed more popularly than ever.
Further, with the development of high-speed liquid crystal device, another color liquid crystal device (scheme) has been proposed.
Generally, a conventional color liquid crystal display apparatus (device) comprises a pair of substrates between which color filters of red (R), green (G) and blue (B) and a liquid crystal are disposed and includes a plurality of pixels each comprising a set of color pixels (sub-pixels) of R, G and B which transmittances are independently controllable. Specifically, the transmittances of the color pixels (R, G, B) are controlled for each color pixel at each corresponding portion of the liquid crystal or in combination with a pair of polarizers, thus ordinarily displaying color images according to the additive process of R, G and B. In that case, as a light source, a transmission-type backlight (unit) emitting white light or a reflection-type light source utilizing an external light may be applicable but their display principals of color space are identical to each other.
Such a color liquid crystal display apparatus is, however, accompanied with a lower efficiency of utilizing light. For example, a white color image is displayed based on the additive process of R, G and B by color-mixing ⅓ (as a wavelength region) of Red (red)-light flux, ⅓ of G (green)-light flux, and ⅓ of B (blue)-light flux, on the basis of light fluxes entering the R-color filters spatially occupying ⅓ of all the incident light. Accordingly, an efficiency of light utilization is merely ⅓ before the incident light enters the liquid crystal layer. This means that a larger power consumption is required of the backlight occupying a major part of all the power consumption of the liquid crystal display apparatus.
Further, for each pixel, three color pixels have to be driven independently. As a result, it becomes difficult to effect a pixel design with an increasing definition, thus lowering an opening rate leading to light utilization efficiency. In addition, from the viewpoint of production costs, the above-mentioned color liquid crystal display apparatus is required to use driver ICs and color filters each with larger bits which are constraint factors to the cost of the liquid crystal display apparatus, thus being disadvantageous.
In view of these circumstances, another type of a color liquid crystal display apparatus has been developed extensively. Particularly, a color liquid crystal display apparatus using a backlight-color switching system as described in JP-A 56-27198 has been actively studied. According to the backlight-color switching system, the color of illumination light (backlight) is switched within a time period of at most the flicker frequency and in synchronism therewith, a (light-)transmission state of the liquid crystal panel is controlled to realize color reproduction by using the spatial additive process. The switching system is also called a RGB field sequential display scheme or field sequential color scheme.
FIG. 6A shows an embodiment of a light emission state at a pixel of a hold-type liquid crystal display apparatus and FIG. 6B shows an embodiment of a light emission state at a pixel of an impulse-type display apparatus.
Referring to FIG. 6A, most of the liquid crystal display apparatus, when a certain pixel is placed in a light emission (open) state, the pixel holds a relatively constant luminance until a subsequent field period (frame period), thus continuing display. On the other hand, in a CRT display of an impulse-type as shown in FIG. 6B, a change in light emission with time is caused instantaneously to provide a high luminance. As a result, at a certain pixel, an instantaneous light emission state is observed one time within one field. At that time, the light emission period varies depending on a characteristic and a resolution of the CRT used.
In the impulse-type liquid crystal display apparatus, when a display image in n-th frame period is changed to that in n+1-th frame period, a sufficient non-display period is ensured before and after the light emission for each frame, thus obtaining displayed data smoothly on the retina.
On the other hand, in the case of the hold type liquid crystal display apparatus, however, even when the liquid crystal device used has a quick response speed, a display image in n-th frame is continuously displayed immediately before the n-th frame period is changed to n+1-th frame, thus leading to blur at an image contour portion or a judder disturbance (such a phenomenon that movement of the image becomes jerky and is observed unnaturally).
Accordingly, although the image deterioration due to double display (simultaneous display) over plural frames can be obviated by the use of a liquid crystal device with a high response speed, the blur at an image contour portion and/or the judder disturbance due to double image (continuous display) resulting from persistence of residual light (or afterglow) on the retinas (human eyes) cannot be removed.
In order to obviate the difficulties, a display period percentage (display duty) (a percentage of a display period to the display period and a non-display period) of the liquid crystal display device via the light source constituting the liquid crystal display apparatus is lowered to provide a non-display period, thus allowing cancellation of image data in a previous frame remaining on the retina to improve clearness of motion images.
In order to decrease the display duty, for example, a lighting period percentage (lighting duty) (a percentage of a lighting period to the lighting period and an extinction (turn-off) period) of the light source per se is lowered to xc2xd by driving the liquid crystal panel (device) at a double speed. As a result, the display duty is also lowered to xc2xd, thus allowing display of clear motion images.
However, the lowering in display duty is accompanied with a problem in terms of display luminance.
Specifically, in the case where a cold cathode tube or an LED (light emitting diode) device allowing high-speed responsiveness is used as a light source for a liquid crystal display apparatus, it is possible to turn off the light in an extinction period, thus resulting in a substantially equal light utilization efficiency. On the other hand, a luminance allowing display or a time opening rate in the liquid crystal display apparatus is lowered depending or the display duty. As a result, in order to realize a display luminance obtained at least at a display duty of 100%; it is necessary to increase a luminance of the light source although the luminance of the light source varies depending on use or specification of the liquid crystal display apparatus.
As a means for increasing the light source luminance, an increase in number of light source unit may be considered. However, the increase of light source unit is accompanied with problems such as increase in space and cost.
As another means, it is possible to increase the light source luminance by increasing a driving current of the light source. Generally, the cold cathode tube or LED as the light source is liable to lower its luminous efficiency (in terms of power consumption) due to high-luminance emission of light except for light with a wavelength above 600 nm.
FIG. 7A is a graph showing a relationship between a relative luminance and a forward current of a LED-type light source and FIG. 7B is that of a cold cathode tube-type light source.
Referring FIG. 7A, four curves indicate current-luminance characteristics of respective color light sources of four colors (red, green, blue and white). The white light source corresponds to a light source emitting a white light obtained by color-mixing lights of red, green and blue. Herein, a maximum value of the relative luminance of white light source (i.e., 3.0 at 100 mA) is defined as a xe2x80x9cmaximum luminancexe2x80x9d. When a luminous efficiency of the white light source (slope of the curve thereof) up to ca. 20% of the maximum luminance (i.e., up to the relative luminance of ca. 0.6) is taken as 100%, the white light source will provides a relative luminance of 5.0 by extrapolation at the luminance efficiency of 100%. In this case, the white light source (with the luminance efficiency of 100%) requires a forward current of 30 mA for a relative luminance of 1.5 (50% of the maximum luminance). However, the white light source used merely provides a relative luminance of 1.2 at 30 mA, thus resulting in a luminance efficiency of ca. 80% ({fraction (1.2/1.5)}). Further, the white light source at the maximum luminance provides a luminous efficiency of ca. 60% ({fraction (3.0/5.0)}). As a result, it has been found from FIG. 7A that the luminous efficiency in terms of power consumption is liable to be lowered with an increasing relative luminance (i.e., with the approach of the maximum luminance).
Further, as shown in FIG. 7B, in order to obtain 50% of a maximum luminance, a forward (tube) current is ca. xc2xc of that required for providing the maximum (relative) luminance.
As described above, in the case where a current providing a maximum luminance is caused to pass through a light source allowing display with high-speed emission of light, when the light source is used in such a state that light emission is performed at a maximum luminance, it is possible to improve a resultant luminance level but the light source is accompanied with a problem regarding power consumption.
An object of the present invention is to provide a liquid crystal display apparatus improved in quality of motion (picture) images while suppression power consumption.
Another object of the present invention is to provide a driving method for the liquid crystal display apparatus.
According to the present invention, there is provided a liquid crystal display apparatus, comprising:
a liquid crystal display device which comprises a pair of electrodes and a liquid crystal and is driven in a succession of frame periods by applying a voltage to the pair of electrodes,
a light source capable of emitting light while changing a lighting duty in a frame period, and
control means for controlling the light source so as to provide a constant time-integrated luminance in each frame period over the succession of frame periods regardless of the change in lighting duty.
In the liquid crystal display apparatus of the present invention, the apparatus further comprises motion detection means for effecting judgment and detection as to whether an inputted digital image signal is for a still image or an motion image, and the control means may preferably comprise light source lighting duty selection means for setting the lighting duty of the light source to 100% when the inputted digital image signal for the still image is detected through the judgment and for setting the lighting duty of the light source to a lower value when the inputted digital image signal for the motion image is detected.
Further the liquid crystal display apparatus of the present invention may preferably comprise motion detection means for effecting judgment and detection as to whether an inputted digital image signal is for a still image or a motion image and a luminance detection means for detecting and comparing an average luminance level over an entire picture area of the liquid crystal display device with a luminance level at a portion where a motion detection of the motion image is effected based on the inputted digital image signal by the motion detection means; and the control means may preferably comprise light source lighting duty selection means for setting a lighting duty of the light source. In this case, based on a result of comparison by the luminance detection means, the lighting duty may preferably be set to a prescribed value by the light source lighting duty selection means.
In a preferred embodiment, the liquid crystal display apparatus may desirably comprise a motion detection means for effecting judgment and detection as to whether an inputted digital image signal is for a still image or a motion image and a luminance detection means for detecting and comparing an average luminance level over an entire picture area of the liquid crystal display device with a luminance level at a portion where detection to a larger degree of movement is effected based on the inputted digital image signal by the motion detection means. In this case, the lighting duty may preferably be set to a lower value with a larger change in luminance between the average luminance level and the luminance level at the portion.
In the liquid crystal display apparatus of the present invention, the liquid crystal display device may preferably be free from a color filter and the light source is capable of emitting three primary colors in synchronism with the liquid crystal display device, and the liquid crystal display apparatus may preferably comprise a planar-sequential color liquid crystal display apparatus for effecting color display according to a field-sequential color scheme in which one frame includes a display period for switching respective colors of the primary colors of the light source in a time sequential manner and is divided into a plurality of fields for controlling transmission and reflection states of the liquid crystal display device in synchronism with the switching of the colors of the light source, thereby to effect color display based on a timewise additive process.
The liquid crystal display apparatus of the present invention may preferably comprise a modulation means for modulating an extraction rate of color-mixing signals. In this case, the lighting duty may preferably be set to a prescribed value by adjusting the modulation means.
The liquid crystal display apparatus may preferably comprise a motion detection means, a luminance detection means, a modulation means and a selector means. In this case, the selector means may preferably effect switching between an automatic mode for determining the lighting duty by judgment as to a still image and a motion image based on a digital image signal inputted by the motion detection means and the luminance detection means and a manual mode for charging the lighting duty by adjusting the modulation means.
In the liquid crystal display apparatus of the present invention, the liquid crystal display device may preferably display an image in a frame period divided into three field periods when an inputted digital image is for a still image and in a frame period divided into at least four field periods when an inputted digital is for a motion image, and the light source may preferably control its lighting state in one frame period so that lighting is effected in a set of three field periods consisting of a red field period, a green field period and a blue field period and the frame period includes an extinction period other than the three field periods.
According to the present invention, there is also provided a driving method for a liquid crystal display apparatus, comprising:
driving a liquid crystal display device comprising a pair of electrodes and a liquid crystal disposed therebetween by applying a voltage to the pair of electrodes in a succession of frame periods,
turning on a light source capable of emitting light while changing a lighting duty in a frame period, and
controlling the light source so as to provide a constant time-integrated luminance in each frame period over the succession of frame periods regardless of the change in lighting duty.
According to the liquid crystal display apparatus of the present invention, it becomes possible to display an image at a desired display duty by changing appropriately the display duty depending on image data as to whether the display image is a motion image or a still image. Further, a display luminance of the liquid crystal display apparatus is controlled by making reference to a display luminance at a minimum display duty. As a result, e.g., it is possible to effect display with a constant luminance in time integration value even when the display duty is changed.
When data writing in a liquid crystal panel (device) is performed according to the raster scanning (sequential writing) scheme or when color display is performed according to the planar sequential scheme, timing of lighting of the light source is controlled in synchronism with the drive of the liquid crystal panel according to the raster scanning scheme or the planar sequential scheme, thus ensuring a constant display luminance irrespective of the display duty.
Further, when the still image is displayed, a display study at that time is 100%. At that time, a luminance level of the light source is xc2xd of that in a maximum light emission state, thus resulting in ca. {fraction (3/10)} of power consumption. Accordingly, a luminous efficiency is increased up to 1.66 ((xc2xd)xc3x97({fraction (10/3)})) times that in the maximum light emission state.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.